Weeks 19 and 20 of the Early Operations system optimization period
Week 19 included a combination of closed-dome engineering, on-sky engineering tests, and preparations to begin a period of sustained pre-LSST observations using a Feature Based Scheduler (FBS) configuration similar to the baseline (v5.1) LSST survey strategy. During Week 20, the team decided to temporarily pause the sustained pre-LSST observations to gather further diagnostic data for the delivered image quality.
Over the past several months, as the overall stability of the system has improved and more diagnostic information has become available, the team has observed that a majority of visits still show larger than expected ellipticity relative to design requirements. In addition, a fraction of visits exhibit gradients of the PSF size and shape across the field of view.
To address these challenges, during Week 20, the team conducted multiple full-night tests to evaluate the convergence and stability of the delivered image quality when running the Active Optics System (AOS) closed loop at a fixed telescope pointing (elevation, azimuth, rotator angle, filter). These tests have been designed to isolate and characterize specific aspects of the delivered image quality, using a slightly different configuration for the testing on each night. While the typical measured PSF FWHM during these tests has been ~0.8" and generally stable throughout the night, there is persistent high ellipticity (median of per-sensor ellipticity values ~0.08, with tails extending to ~0.15), and periods with more complex observed PSF shapes as well as gradients of the PSF shape across the field of view. The team continues to pursue both offline data analyses and is designing additional on-sky tests to evaluate the wavefront measurement, prediction of the PSF shape across the focal plane, application of optics corrections, and potential non-optics effects that impact the PSF shapes.
During the current period with an emphasis on on-sky engineering, there will be fewer observations of fields with available template coverage that would generate alerts.
Additional on-sky tests during this period included densely dithered star field observations of the moderate stellar density field Rubin_SV_225_-40 that had also been extensively observed during the on-sky commissioning period. This test is to evaluate the internal astrometric and phometric solutions following the thermal cycle of LSSTCam in February and the sequencer CCD readout controller firmware update to adopt the natural frequency of the readout electronics.
The team also conducted further stray light tests to confirm that the completed extension to the mid-level baffle fully blocks unwanted stray light paths.
A filter swap to exchange y for u occurred on 10 March. The currently installed filter set is ugriz, with the next filter swap to exchange u for y planned for 24 March.